Computer applications store data in file systems or raw volumes without knowledge of the underlying structure of the storage object. This abstraction allows virtualization of the storage so that redundancy and other storage features can be implemented transparently to the application. However, if the application maintains metadata for checking the integrity of the data when it is read from the storage object, it does not have a mechanism for retrieving a redundant copy of the data. For example, an application could generate a cyclic redundancy code (CRC) and append it to the data written to a mirrored volume or disk array that implements mirroring. The fact that the data is mirrored is transparent to the application. Under normal conditions, the volume manager or disk array would detect any errors that occur and retrieve the data from the mirrored copy, returning it to the application without any indication of error. If the application should detect that the CRC on a read block of data is incorrect, it does not have the capability to read the alternate data image to check it for accuracy.
Unfortunately, the existing solutions to this problem are to retry the read operation or recover data from a back-up device. A retry of the read operation could be attempted, hoping that a subsequent read might be executed to the mirror image of the data instead of the primary storage location. The most likely outcome would be that the same data image would be returned from some underlying cache memory. In the case of RAID 5, where the only alternate data image available must be reconstructed from the other drives in the redundancy group, the alternate image would never be accessed unless the primary copy indicated an error to the disk array.
Therefore, it would be desirable to provide a mechanism to recover from certain types of errors without going through the tedious off-line process of restoring data from a back-up device.